This invention relates to nuclear-reactor power plants and has particular relationship to the control of the reactivity of nuclear reactors by displacement of the neutron-energy spectrum so as to enhance the flux of thermal neutrons. The effect of this displacement is to increase the number of neutrons whose energy is in the thermal range and to decrease the number of neutrons whose energy is in the epithermal range. The neutrons whose energy is in the thermal range are in thermal equilibrium with the coolant in the reactor. This invention deals predominantly with reactors whose coolant is light water, an effective moderator. To the extent that this invention is applicable to reactors with other coolants, for example, reactors whose coolant is deuterium oxide, such application is within the scope of this invention.
In addition to involving reactors whose coolant is light water, this invention involves reactors whose core includes sealed tubes containing a gas which may be helium or any other gas which does not deleteriously affect the reactor. The moderation characteristic or property of the gas is different and substantially less than that of the coolant. To change the moderation of the reactor, the tubes are ruptured and the gas in the tubes is replaced by coolant increasing the moderation. Such a reactor is disclosed in application U.S. Pat. No. 4,371,495, granted Feb. 1, 1983 to George R. Marlatt and assigned to Westinghouse Electric Corporation (herein Marlatt patent). The Marlatt patent is incorporated by reference in the instant application. In the reactor disclosed in Marlatt patent, sealed gas-filled tubes are interposed in all or some fuel assemblies and are mounted between the upper and lower nozzles of each assembly. The upper nozzle of each assembly which includes the tubes carries spikes or knives each displaced a short distance from the top of a corresponding tube. The tubes are automatically ruptured in the course of operation of the reactor. As the reactor operates and the tubes are bombarded by neutrons, they swell. Ultimately, the tubes reach a height such that their tops are engaged by the spikes and are ruptured. Coolant replaces the gas in the ruptured tubes and the moderation of the neutrons is increased.
The tubes in Marlatt's reactor swell at diferent rates under neutron bombardment so that the changes in moderation are moderate and a high thermal power increase does not occur. It is desirable that the change in moderation by rupture of tubes be controlled methodically by selective commands of an operator with the reactor system appropriately set to avoid excessive reaction to the interruption. It is an object of this invention to achieve this objective and to provide a nuclear-reactor system whose moderation is changed by rupture of tubes in the core but wherein the rupture of the tubes shall be controlled by control command facilities outside of the reactor pressure vessel. It is required that the rupturing be accomplished with a minimum penetration of the pressure vessel and that the rupturing mechanism be failsafe. It is also desirable that the mechanism be capable of being installed in reactors in current operation.